Radiotherapy apparatus involves the production of a beam of ionising radiation, usually x-rays or a beam of electrons or other sub-atomic particles. This is directed towards a cancerous region of the patient, and adversely affects the tumour cells causing an alleviation of the patient's symptoms. Generally, it is preferred to delimit the radiation beam so that the dose is maximised in the tumour cells and minimised in healthy cells of the patient, as this improves the efficiency of treatment and reduces the side effects suffered by a patient. A variety of methods of doing so have evolved.
One principal component in delimiting the radiation dose is the so-called “multi-leaf collimator” (MLC). This is a collimator which consists of a large number of elongated thin leaves arranged side to side in an array. Each leaf is moveable longitudinally so that its tip can be extended into or withdrawn from the radiation field. The array of leaf tips can thus be positioned so as to define a variable edge to the collimator. All the leaves can be withdrawn to overlap the radiation field, or all the leaves can be extended so as to close it down. Alternatively, some leaves can be withdrawn and some extended so as to define any desired shape, within operational limits. A multi-leaf collimator usually consists of two banks of such arrays, each bank projecting into the radiation field from opposite sides of the collimator.
In addition, radiotherapy apparatus is now often integrated with a diagnostic x-ray source and a two dimensional flat panel detector, that are together able to carry out online CT scanning during treatment of a patient, or immediately before such treatment. Previously, investigative procedures were carried out in order to determine the position of the tumour and then treatment plans were prepared in order to design a dose profile that would apply the desired dose to the tumour cells and a minimum dose to surrounding patient cells. Online CT scanning during treatment allows for the treatment plan to be revised during a treatment, and it has therefore been proposed in principle that there should be feedback from the results of the CT scanning to the control unit setting the MLC leaf positions, so that the radiation beam is delimited to the current actual position of the tumour rather than a previous position at the time when diagnostic procedures were carried out.